1. Field of the Invention
The present invention relates to a sensor used for observations of the topography of sample surfaces and various physical properties (such as viscoelasticity) under liquid environments by making use of a self-detecting probe. The invention also relates to an observation apparatus for use in liquid environments.
2. Description of the Related Art
In recent years, with development of the nanotechnology, techniques for imaging and observing the surfaces of samples such as bio samples and semiconductor samples at high resolution have been required. One known apparatus for realizing this requirement is a scanning probe microscope (SPM). The scanning probe microscope is an instrument capable of imaging very tiny regions on the surfaces of various samples of metals, semiconductors, ceramics, resins, polymers, biomaterials, and insulators and of enabling observation of the surface topography of the sample and various physical properties such as viscoelasticity at atomic-level high resolution. In addition, the scanning probe microscope can be used under various environments such as in a vacuum, in a gas, within the atmosphere, and in liquids. Hence, scanning probe microscopes are adapted to be used in a wide range of applications.
Especially, in recent years, there is a very strong need for a technique enabling observation of samples under liquid environments because observation of the processes of electrochemical reactions of a sample in progress at the interface with an electrolytic solution or in vivo observation of a bio sample within a culture solution will be an important theme in forthcoming experiments or researches. Usually, when observation is made in liquids, an atomic force microscope (AFM) that is one type of scanning probe microscope is used. The atomic force microscope is used to observe the surface topography of a sample or its various physical properties by bringing a probe tip attached to the front end of a cantilever into contact with or close to the surface of the sample and performing scanning while controlling the distance between the probe tip and the sample such that the amount of flexure of the cantilever is kept constant.
In order to measure the amount of flexure of a cantilever, it is customary to use a so-called optical lever. In this method, the rear surface of the cantilever is illuminated with laser light. Laser light reflected from the rear surface is detected by a detector. The amount of flexure of the cantilever is measured based on a variation (variation in optical path) in the position at which the laser light is received. Therefore, it is necessary that the laser light be precisely directed at the rear surface of the cantilever. In addition, it is necessary to prealign the laser light path to assure that the reflected light enters the detector. That is, preparations for an observation are necessary. However, the prealignment is a very difficult work to perform because the optical path is aligned relative to the cantilever having a diameter of as small as tens of micrometers. Consequently, much labor and time are required. Especially, where observations are made under liquid environments, the cantilever is submerged in a liquid, making the work difficult.
Moreover, the cantilever is a consumable part. Therefore, it is necessary to replace it at appropriate times. When the cantilever is mounted in a normal manner, the worker picks up the body (about several millimeters in length) supporting the cantilever with tweezers and sets the body onto a cantilever holder. Unfortunately, this work itself is difficult for a novice to perform. For this reason, the present situation is that when the cantilever is mounted, the cantilever is damaged frequently. In addition, if the cantilever has been mounted successfully, the work for aligning the laser light path as described previously must be performed subsequently. Consequently, very cumbersome preparative steps must be carried out until an observation or measurement is started in practice.
In recent years, a self-detecting probe including a cantilever in which a strain resistive element (displacement sensor) such as a piezoelectric device is incorporated has been developed (see U.S. Pat. No. 5,345,815). The self-detecting probe is briefly described with reference to some drawings.
As shown in FIG. 20, a self-detecting probe, generally indicated by reference numeral 100, has a cantilever 101 and a body portion 101b. In addition, the probe 100 has a strain resistive element 102 whose resistance value varies with the amount of flexure of the cantilever 101 and a support base 103 supporting the body portion 101b. The cantilever 101 has a probe tip 101a at its front end. The base end of the cantilever 101 is supported to the body portion 101b. 
The probe tip 101a, cantilever 101, and body portion 101b are integrally formed from a semiconductor material such as silicon. The strain resistive element 102 is located near the base end of the cantilever 101 and fabricated by ion implantation or other technique. Interconnects 104 are electrically connected with the strain resistive element 102 and extend from the cantilever 101 to over the body portion 101b. The interconnects 104 are electrically connected via connecting wires 105 with an interconnect pattern 106 formed on the support base 103. The ends of the interconnect pattern 106 form input/output terminals for sensor signals.
The self-detecting probe 100 constructed in this way makes it possible to monitor the resistance value of the strain resistive element 102 via the input/output terminals. The amount of flexure of the cantilever 101 can be precisely known based on the variation in the resistance value. Therefore, the self-detecting probe 100 dispenses with the use of the aforementioned optical lever.
In particular, the development of the self-detecting probe 100 can dispense with the work for aligning the laser light, which has been heretofore difficult to achieve. Consequently, many persons can enjoy the merit, and even novices can treat atomic force microscopes relatively easily. The emergence of the self-detecting probe 100 permits the consumable cantilever 101 to be replaced relatively easily in succession even in atomic force microscopes which are used in industrial applications and which enable automated measurements. In this way, the self-detecting probe 100 has contributed to development of the leading edge industry.
Although the above-described self-detecting probe 100 is easy to handle and considered to be a forthcoming mainstream technique, the probe tip is not adapted for use in observations under liquid environments.
First, when the self-detecting probe 100 that has been put into practical use and contributed to the development of the industry is used in practice, the probe is mounted on a substrate 110 made of a resin material such as glass epoxy to facilitate handling as shown in FIG. 21. A conductive pattern 112 electrically connected with the ends of the interconnect pattern 106 via wires 111 is formed on the substrate 110.
The substrate 110 needs to be set on a cantilever holder 120 shown in FIG. 22. The cantilever holder 120 consists chiefly of a holder body 122 to which a resilient electrode 121 is mounted and an electrode guide 124 firmly mounted to the holder body 122. The guide 124 supports the front end of the resilient electrode 121. A space into which the substrate 110 is inserted is created between a cantilever base 123 and the electrode guide 124.
When the substrate 110 is set in the cantilever holder 120 constructed in this way, the substrate 110 is inserted into the gap between the electrode guide 124 and the cantilever base 123 while orienting the conductive pattern 112 toward the resilient electrode 121. Because the substrate 110 is held down by the resilient electrode 121, the substrate 110 is held stationary. At the same time, the resilient electrode 121 makes contact with the conductive pattern 112, securing electrical connection. As a result, a flexure signal from the cantilever 101 can be picked up. The function of the atomic force microscope can be exhibited.
When observations are made within the atmosphere, no problems arise. However, when observations are made under liquid environments, it is inevitable that the self-detecting probe 100 itself is submerged in a liquid. Therefore, the interconnects 104, interconnecting wires 105, and interconnect pattern 106 are similarly submerged in the liquid. Consequently, electrical current leaks at the locations where they contact the liquid. In the worst case, electrical shorting takes place. Therefore, noise tends to enter the displacement signal. It has been impossible to precisely measure the displacement of the cantilever 101. Furthermore, electrochemical reactions induce corrosion at the locations where they contact the liquid. Hence, damage and breaks in conductors tend to occur. For these reasons, the self-detecting probe 100 is unsuitable for observations under liquid environments.
In addition, depending on the imaging mode, a piezoelectric device may be interposed between the cantilever holder and the self-detecting probe 100, and the cantilever 101 may be vibrated. In this case, however, the piezoelectric device itself also touches the liquid. Therefore, the piezoelectric device tends to suffer from similar problems.